The goal of this proposed research is to characterize and to understand the molecular mechanism of (Na+ + K+)-ATPase, also known as the sodium pump, in terms of its transport properties and its organization within the membrane. Thus, the approach of this proposal is to obtain information on the fast ion transport kinetics and ultrastructure of (Na+ + K+)-ATPase/model membrane systems using electrical relaxation, freeze-fracture-etch and x-ray diffraction techniques. Both isolated native membrane and reconstituted liposomes containing the purified ATPase from pig kidney outer medulla will be fused with solvent-free planar bimolecular lipid membranes (BLMs) for voltage jump current relaxation experiments which will help to evaluate the rate constants of ion translocation steps in the coupled reactions. The time constant, the instantaneous current, and the total amount of charge transferred during a current response after a voltage step will be determined as functions of membrane lipid composition and temperature. The voltage dependence and teh time course of current response should provide us insight regarding the construction and coupling of the ATPase ion conducting pathways. At the same time, these native membranes and liposomes containing ATPase will be analyzed to determine the mass distribution of the protein in the membrane by negative staining, x-ray diffraction, and high vacuum freeze-fracture etch experiments. The hydrophilic surfaces and hydrophobic interiors of ATPase/BLMs will be examined by negative staining and freeze-fracture-etch techniques, respectively. Comparative ultrastructural studies of BLMs and native membrane containing ATPase should be useful in determining if the original topography and spatial relationships of the pump components have been restored during the recoinstitution process.